<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>For循环优化</title>
  </head>
  <body>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>
    <p class="btn">add</p>

    <script>
      var aBtns = document.getElementsByClassName("btn");

      for (let index = 0; index < aBtns.length; index++) {
        console.log(index);
      }

      for (let index = 0, len = aBtns.length; index < len; index++) {
        console.log(index);
      }
    </script>
  </body>
</html>
